The objective of the proposed work is to characterize seasonal and chemical diversity of fish ciguatoxins and establish biomarkers in the marine toxin food chain. Ciguatoxic barracuda (Sphyraena barracuda) will be collected monthly and the livers of individual specimens extracted and analyzed for toxicity and toxin deversity. These data will be correlated with the stable nitrogen isotope ratios of tissue samples of these barracuda, flesh samples of the doctorfish Acanthurus chirurgus and its algal food sources on the reef, Dictyota sp. and Thalassia testudinum sampled concurrently. The fish scale melanophore assay will be used to trace toxin presence in the algal - doctorfish - barracuda food chain. Ciguatoxins isolated from extracts of Caribbean barracuda have pronounced effects on the coloration of poisoned reef fishes, causing them to blanch or become pale. These effects can be detected in the pigment cells found in isolated fish scales. Recent results indicate that ciguatoxicity may be detected by stable nitrogen isotope analyses. These phenomena may enable us to trace the ciguatoxins as they move through the cituatera food chain and establish a system that will detect these toxins in fish prior to their human consumption.